The present invention relates to a method for preparing carboxylic acids having (n+1) carbon atoms, or the related esters, by liquid phase carbonylation of an alcohol having (n) carbon atoms, in the presence of a homogeneous catalyst including iridium.
More particularly, the method according to the invention enables obtaining a high productivity of carboxylic acid formed with an improved selectivity in this product.
During carboxylic acid preparation by reaction of carbon monoxide with at least one alcohol, two catalytic cycles are used, the first is constituted by the carbonylation reaction as such, and the second by a secondary reaction, commonly called water gas reaction. During this reaction, gaseous by-products are formed such as notably carbon dioxide.
Thus, the object to be achieved is obviously to favour the first reaction over the second.
The carbonylation of an alcohol, or any other carbonylatable compound, in the presence of an iridium-based catalytic system giving the corresponding carboxylic acid or ester is known to the person skilled in the art. However, such methods are not very efficient since the rates of carbonylation that are reported are in the order of 2 to 4 mol/hour.liter of acid formed.
In the French patent application No. 93 03734, a method of carbonylating methanol is described which uses a catalytic system which comprises iridium. This method is characterised by the maintenance of a specific composition of the reaction mixture. In the case of obtaining acetic acid from methanol, the composition comprises up to 10% water, alcohol and methyl iodide; up to 40% methyl acetate; the rest being constituted by acetic acid which is used as reaction solvent. The tests described have been carried out with relatively low partial carbon monoxide pressures, in the order of 10 to 20 bar. The performances of this method are appreciably improved over those of prior methods, and the technical interest of the latter is not questioned. However, the selectivity in by-products such as carbon dioxide remains high, since it ranges between 1 and 2%.
Furthermore, a carbonylation method is described in the French patent application No. 94 05896 which uses a catalytic system comprising iridium and iodides which are soluble in the reaction medium under the reaction conditions, and which are notably selected from alkali metal iodides. The performances, in terms of rate of carbonylation are there further improved over the prior methods, but the problem of the selectivity in carbon dioxide always exists. In fact, the selectivities in this by-product range between 1 and 3%.
Such values are not negligible in this sense that they correspond to relatively significant losses of carbon monoxide; losses which are felt in two ways. First of all, and obviously, the carbon monoxide employed is consumed for something other than the formation of the acid or the ester. Further, an additional loss is noted in carbon monoxide, due to the necessity of increasing the number of purges and consequently increasing the losses in carbon monoxide brought about during the evacuation of the gases. In fact, the appearance of gaseous by-products contributes to the lowering of the partial carbon monoxide pressure in the gaseous blanket of the reactor, resulting in the necessity of increasing the number of purges in order to maintain said partial pressure constant.